The present invention pertains to electrode structures in systems constructed of data storage elements which employ an ionizable gas to address an array of such storage elements.
U.S. Pat. No. 4,896,149, issued Jan. 23, 1990 (xe2x80x9c""149 Patentxe2x80x9d) of Buzak et al. for xe2x80x9cAddressing Structure Using Ionizable Gaseous Medium,xe2x80x9d which is assigned to the assignee of the present invention, discloses an addressing structure using an ionizable gaseous medium. Such an addressing structure may be used in a system constructed of data storage elements and addressing those data storage elements with the use of an ionizable gas. Examples of such systems are flat panel displays, video cameras, and memory systems.
The system disclosed in the ""149 Patent has an electrode structure having rows of plasma channels, each of which is filled with an ionizable gas. Extending along the base of each of those channels are a row electrode and a reference electrode. The row electrode is electrically driven as a cathode, and the reference electrode is referenced to ground and acts as an anode when the row electrode is electrically driven as a cathode.
The walls between the channels define mesa-like supporting structures having sloping walls and flat tops, upon which rest a thin dielectric material that seals the channels. The channels are filled with an ionizable gaseous medium contained at a low pressure. To achieve a dense addressing structure, such as is required for a high resolution flat panel display, the optically inactive space between the channels is small compared to the width of the channels. The supporting structures, therefore, are thin and, consequently, fragile and easily damaged.
A second electrode structure is spaced apart from the layer of dielectric material. The space between the dielectric layer and the second electrode structure is filled with an electro-optical material, such as a nematic liquid crystal. The liquid crystal is contained at a pressure matching that of the gaseous medium within the channel to reduce pressure on the thin dielectric and thereby prevent it from deforming or breaking. Because small variations in thickness affect the optical properties of the liquid crystal, the size and uniformity of the spacing between the dielectric layer and the second electrode structure are critical when the addressing structure is used in a liquid crystal display.
Several methods have been developed for maintaining the spacing between electrodes of prior art liquid crystal panels. Such panels typically comprise a liquid crystal material sandwiched between two glass plates that are thick relative to the dielectric layer used in the addressing structure in the ""149 Patent. The spacing between the glass plates is typically maintained by particulate spacers, typically in the form of fibers or beads made of black or clear glass or plastic. The particulate spacers are blown onto one of the glass plates and are, therefore, randomly located. The density of the spacers is sufficiently low that the spacers do not significantly detract from the displayed image, but sufficiently high to provide adequate support for the second plate.
Spacing layers have also been applied as a coating and then photoetched. Examples are described in U.S. Pat. No. 1,336,254 of Gunther for xe2x80x9cDevices,xe2x80x9d which uses a photoresist layer as a spacer and sealer, and Japanese Pat. No. 1,550,155 of Lin-Hendel et al. for xe2x80x9cimproved Liquid Crystal Display Incorporating Photopatterned Spacers,xe2x80x9d which is assigned to the assignee of the present invention and which uses photopatterned polyimide spacers in a conventional liquid crystal panel. These methods have been generally abandoned in favor of particulate spacers, which are easier to use and produce more repeatable spacings.
In prior art liquid crystal panels, the liquid crystal layer is typically positioned between two glass plates having electrodes on their inner surfaces. In the addressing system described in the ""149 Patent, the liquid crystal layer is positioned between the thin dielectric layer and the second electrode structure. Voltage applied across a display element is stored partly across the liquid crystal layer and partly across the thin dielectric, but only the voltage stored across the liquid crystal layer affects its electro-optical properties. Reducing the thickness of the thin dielectric layer increases its capacitance, thereby reducing the fraction of the applied voltage that is stored across the dielectric layer and increasing the voltage stored across the liquid crystal. To reduce drive voltage requirements, the dielectric layer is made very thin, so that more of the applied voltage is stored across the liquid crystal. The thin dielectric is supported on the narrow top surfaces of the supporting structures separating the channels, resulting in an assembly that can be easily deformed or damaged.
The use of blown-in particulate spacers creates problems when used with the addressing structure of the ""149 Patent. Because the dielectric layer is thin and unsupported between the channel walls, the dielectric tends to deform under a localized load, such as the load applied to the dielectric layer through the blown-in spacers. Because the liquid crystal is maintained at below one atmosphere of pressure, there is a net pressure on the second electrode structure, which tends to transfer the pressure through the spacers and onto the dielectric layer, deforming and possibly breaking it. Blowing in an adequate number of spacers to spread the force over a sufficiently wide area would intrude too many spacers into the display area and degrade the image.
An object of the present invention is, therefore, to provide for an electro-optical addressing system a method and an addressing structure that maintains a layer of an electro-optical material at a uniform, predetermined thickness.
Another object of this invention is to provide such an electrode structure and method that do not degrade a displayed image.
A further object of this invention is to provide such an electrode structure and method that do not deform or damage the dielectric sheet.
This invention relates to a method and apparatus for maintaining a layer of an electro-optical material, such as liquid crystal, at a uniform and accurate thickness in an addressing structure similar to the one described in the ""149 Patent. The invention uses spacers that are formed in inactive areas of the display, particularly in areas that are directly above the top surfaces of the support structures of the first electrode structure. By so locating the spacers, the force transferred from the second electrode structure through the dielectric layer is transmitted directly onto the support structures. Therefore, the dielectric layer is less likely to deform or break. Furthermore, the number of spacers required is reduced compared to the number of random, blown-in spacers that would be needed because the spacers are positioned advantageously to support the second electrode structure.
Particulate contamination on the top surface of the support structures can hold the thin dielectric layer off of the support surface in places, forming a local tent-like structure. If a spacer were to press on the dielectric layer at such a structure, the dielectric layer may break or form defects. Minimizing the number of spacers used reduces the probability of a spacer pressing on a tent-like structure and thus reduces such breakage or defects.
A first preferred embodiment entails the use of photoetched polyimide spacers positioned at intersections between first inactive areas defined by the top surfaces of the support structures and second inactive areas defined by the spaces between the column electrodes. The spacers are not needed at every intersection to provide sufficient support for the second electrode structure, and, therefore, spacers are used at fewer than all of the intersections to reduce contamination-induced breakage.
A second preferred embodiment entails the use of particulate spacers, such as glass fibers, entrained in a xe2x80x9cblack surroundxe2x80x9d material that is used to mask areas of uncontrolled optical properties in the display. When the black surround material is photoetched, spacers in active areas of the display are removed together with the black surround, while spacers in inactive areas remain held in place by the black surround.
A third preferred embodiment entails forming spacers from the black surround material. The black surround is patterned to create thick areas of black surround that function as spacers and thin areas of black surround that provide only the conventional masking function and allow the liquid crystal material to flow between display elements.
Additional objects and advantages of the present invention will be apparent from the detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.